The giant ice planets of the solar system, Uranus and Neptune, are similar in many ways, no doubt about that, but also very different from each other. Astronomers have spent a lot of time trying to figure out where all these peculiarities come from, and now a new study exposes that the source of these peculiar characteristics is two different giant impacts.
How are they alike
They seem to look the same. They have similar masses, sizes, and distances to the sun, and scientists have long believed that they have formed similarly as well. Of course, we just have to look at them to see that they are two clearly different planets.
“However, there are also notable differences between the two planets that require explanation,” says Christian Reinhardt, who together with Alice Chau, Joachim Stadel and Ravit Helled, members of PlanetS from the University of Zurich (Switzerland), studied both planets. “For example, Uranus and its main satellites are tilted about 97 degrees in the solar plane and the planet effectively rotates retrograde with respect to the Sun “, clarifies Joachim Stadel.
Probably the most obvious difference between the ice giants is their angle of rotation since, unlike Uranus, Neptune and most of the planets in the solar system have rotations that are roughly aligned with their orbits (although Venus also rotates in the “wrong” direction).
For the new study, published in the journal Monthly Notices of the Royal Astronomical Society , the scientists conducted a series of computer simulations that showed that a series of collisions with large, rocky bodies would have been responsible for each of the stories of the two planets, which would explain why they look so disparate at first glance. A unique and traumatic event marked their history at some point in the past of these outer planets, the outermost of the solar system.
Difference in satellites
Astronomers know that Uranus must have been hit hard in the past since it orbits sideways, but what about Neptune?
The main satellites of Uranus are in regular, inclined orbits with the planet, suggesting that they formed from a disk, similar to Earth’s Moon. Instead, Triton, Neptune’s largest satellite, is very tilted and therefore most likely an object captured by gravity . Neptune’s other moons are also captured, they are irregularly shaped and appear to have an internal heat source.
Planetary scientists ran simulations to test how collisions could diversify planets by starting out similarly. Thus, each planet was hit by a planetoid between one and three times the mass of the Earth, a common event in the formation of star systems. The key difference was how those collisions unfolded.
Starting with Uranus and Neptune before impact, they showed that the collision of a body with 1-3 Earth masses on both planets could explain this duality.
“We clearly show that a formation pathway initially similar to Uranus and Neptune can lead to the observed dichotomy in the properties of these fascinating outer planets,” explained co-author Ravit Helled.
Either way, there is still a lot to do, as researchers still need to connect these potential collisions with simulations of the formation of the solar system, among other things.
Referencia: “Bifurcation in the History of Uranus and Neptune: The Role of Giant Impacts,” Christian Reinhardt et al., 2019 Nov. 22, Monthly Notices of the Royal Astronomical Society [https://doi.org/10.1093/mnras/stz3271, preprint: https://arxiv.org/abs/1907.09809].
